The attached FIGS. 1 and 2 show a mold M according to the prior art comprising three parts 1, 2 and 6, the mold M being seen in the assembled state on the ends 4 of two railway rails 3, to be welded to each other. These views are reprised from application FR 2 923 740 in the name of the present applicant.
These ends 4 rest on a base part 6 shown in detail in FIGS. 3 and 4.
The base part 6 (or “briquette”) is generally part of a rectangular parallelepiped and has an upper receiving face 60 and supports of the foot of said rails, this upper face having a hollow cavity 61 comprising a base 610, two opposite longitudinal flanks 611, as well as two opposite transverse flanks 612. Configured in this way, the cavity also has the form of a rectangular parallelepiped.
During molding, the welding material is introduced not only between the rail ends 4, but also into the free space E delimited by the above cavity 6.
On the finished product this is embodied by a region projecting from the lower face of the ends 4 joined to each other. This projection is not disabling to the extent where the rails will rest on sleepers.
The certification of a welding method by aluminothermy needs to satisfy the requisites of a specification. This specification demands respect of many criteria relative especially to chemical analysis, hardness, defect analysis, width of the molten zones, fatigue resistance, flexion resistance, etc.
Depending on the certification body, requisites relative to these different criteria can vary.
Accordingly, in some countries the criterion most difficult to respect is flexion resistance (3 points).
FIG. 5 schematically illustrates a flexion test conducted on rails. As is evident, the couple of rails 3 welded to each other is supported by the foot on two cylindrical surfaces A on either side of the weld and a force F is applied to the rail head, vertically to the weld S. According to the standard EN14730-1 (annex F) the radius r of application of the load and support of the sample is between 25 and 70 mm, the minimal length of the sample is 1150 mm and the load rate is less than or equal to 60 kN/s.
In this way, for rails referenced JIS60A HH (a thermally treated nuance, of high hardness), the certification body imposes rupture displacement of at least 10 mm. Experience shows that, irrespective of developments made, this value is difficult to achieve, as shown in the following table.
Slow Flexion trialSlow Flexion trialFoot in tensionHead in tensionMinimalMaximumMinimalMaximumRailGradedeflectiondeflectiondeflectiondeflection50ND10 mm800kN13 mm800kN50N34510 mm950kN10 mm950kN60KD10 mm1100kN13 mm1100kN60K34510 mm1250kN10 mm1250kN
The grade corresponds to the nuance of the weld. In this way, the Grade 345 corresponds to the nuance used for a rail with hardened head (HH).
To improve flexion resistance, research has focused on analysis of facies, that is, rupture zones and initiation points of ruptures on the welds broken in flexion.
Morphologies and sizes of the molten zones at the foot have been observed. The geometries and dimensions of bosses of molds and finally of bosses of welds have been analyzed.
The general aim was to avoid geometric singularities which penalize the mechanical properties and can cause equally penalizing metallurgical defects.
In this way, against all expectations, the present applicant has designed a particular geometry of a base part which ensures better resistance to flexion forces.